Ballast assemblies for two lamp instant start fluorescent lighting fixtures include primary and secondary coils for maintaining bulb illumination, with a starter coil providing a current boost for first charging the bulb and initiating fluorescence. The coils are all located in a common housing, with terminations and wiring provided for energizing the coils.
In a typical ballast assembly, a thermal circuit protector (TCP) is usually included as a safety device which may sense an abnormal increase in temperature in the assembly. Such a temperature increase is usually attributed to an overload or short circuit condition. The TCP prevents high temperatures from causing damage to the assembly, by interrupting the power supply to the ballast assembly.
The TCP has a bimetallic switch which is calibrated to open at a specified temperature and remain open while the temperature is above the calibrated limit. Upon returning to the set temperature, the TCP bimetallic switch closes, allowing normal operation to commence. TCP's with bimetallic switches are proximity devices which must be placed as close as possible to the heat source to provide quick response. Generally, the TCP is connected in the primary power supply to the transformer such that upon switch activation the TCP interrupts the power supply to prevent further overheating. Besides current overloads and short circuits, two-lamp instant start ballast assemblies have the additional problem of lamp rectification. Under lamp rectification, the current in the starter coil jumps from a nominal 80 milliamperes to approximately 200 milliamperes. To protect the assembly from damage, the starter coil requires a dedicated TCP. Thus, two TCPs are placed in the housing, one adjacent the primary coil and one adjacent the starter coil. Each TCP is wired in series with the power supply. Therefore, if one or the other or both coils are subjected to an abnormal temperature, the bimetallic switches will open, cutting power to the ballast assembly.
One problem with this arrangement is that each coil subject to overheating must have an adjacent TCP, with the number of TCPs increasing the cost of the ballast assembly. In addition, wiring and ballast assembly costs increase with the increase in component parts.